In the wireless communication system, due to factors such as the mobility of a user equipment (UE) and the multi-path propagation of a wireless signal, the channel state between a base station and a UE is changed along with a change of factors such as the time, frequency, and the UE location. Therefore, a measurement of the channel state needs to be performed, then a communication is performed according to the measured channel state information (CSI), so as to ensure the transmission efficiency of the system and improve the quality of service of the UE.
As in the long term evolution (LTE) system and the long term evolution-advanced (LTE-Advanced, LTE-A) system, a UE needs to measure and feeds back various measured values to a base station. For example: in order to support a dynamic scheduling and downlink multi-input multi-output (MIMO) transmission technique, the UE needs to measure the channel state and report the CSI to the base station; in order to support a cell reselection and cell handover, the UE needs to measure and report the reference signal received power (RSRP) and reference signal received quality (RSRQ) to the base station; in order to support an uplink power control, a physical layer of the UE needs to measure and report the RSRP to a high layer of the UE, after the high layer filters the reported RSRP, the physical layer measures a path loss (PL) needed in a uplink power control formula by using the filtered RSRP, and the like.
The reporting of CSI includes a periodic CSI reporting and an aperiodic CSI reporting. Moreover, either the periodically reported CSI or the aperiodically reported CSI needs to be derived by combining the measurement signal and channel interference from the environment. In the LTE system, the measuring of channel state may be based on a cell-specific reference signal (CRS), and may also be based on a channel-state information-reference signal (CSI-RS), i.e., a channel state measurement signal may be the CRS, and may also be the CSI-RS. Carriers in the LTE system are backward-compatible, which may support a LTE UE access of previous versions and be served.
Generally, the CRS is usually transmitted on full frequency band of each downlink sub-frame, and the CSI-RS is periodically transmitted on full frequency band with a fixed period. If the channel state is measured based on the CSI-RS, either for the periodic CSI reporting or for the aperiodic CSI reporting, the base station will periodically transmit the CSI-RS with the fixed period so as measure the channel state, so that the CSI is derived. Therefore, for the aperiodic CSI reporting, the base station will transmit unnecessary CSI-RS, so that unnecessary energies and resources are consumed, not only spectrum efficiency of a system is reduced, but also unnecessary interferences are brought to other cells.
Additionally, no matter for the periodic CSI reporting or the aperiodic CSI reporting, an observation interval of a CSI, which is reported in an uplink sub-frame n, in time domain and frequency domain is not limited, e.g., a UE may average measured channels and/or interferences based on a plurality of sub-frames and/or a plurality of sub-bands so as to derive the CSI, and report to a base station. The UE may also derive the CSI based on channels and/or interferences of one sub-frame and/or one sub-band merely, and report to the base station. When the base station receives the CSI reported by the UE, the base station does not know whether the reported CSI is derived by averaging the channels and/or interferences based on a plurality of sub-frames and/or a plurality of sub-bands, or the reported CSI is derived based on the channels and/or interferences of one sub-frame and/or one sub-band merely. Moreover, when the UE employs an averaging method to derive a CSI, the reported CSI can only represent an average channel characteristic.
In order to solve the above problem, in the LTE system, when a UE is configured with a transmission mode 10, one or more CSI processes may be configured to the UE. One CSI process corresponds to one non-zero power CSI-RS resource and one interference measurement resource (IMR). When the UE is configured with a plurality of CSI processes, different CSI processes may represent different interference environments, so that the base station may derive channel state information in different environments based on the CSI derived through different CSI processes, so as to facilitate the realization of the CoMP transmission mechanism. However, in practical application, due to reasons such as the limited capacity of the UE, some UEs can only be configured with one CSI process. Therefore, a solution is needed to solve the problem that the base station can not derive the CSI of such type of UE in different scenes (or say different interference environments).